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Strong Tornadoes and Flash-Flood-Producing Rainstorms During the Warm Season in the Mid-Atlantic Region

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  • 1 NOAA/National Weather Service Forecast Office, Pittsburgh, Pennsylvania
  • | 2 Department of Meteorology, The Pennsylvania State University, University Park, PA
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Abstract

Storm-related literature spanning 45 years (1942–1986) was examined to 1) establish a climatology of strong tornadoes and intense rainstorms for the Mid-Atlantic region, and 2) identify the key environmental features that distinguish the days producing strong convective events. Ninety-three strong tornado events (F3 or greater) and 63 intense rain events (19 cm or more in 12 h or less) were found for the 45-year period and were investigated.

The diurnal and monthly distributions of the tornado and rainstorm events correspond well with other known climatological studies of similar events. Analysis of the upper-level flow patterns during the events shows that the Mid-Atlantic region is affected by two distinct regimes during the warm season. Events near the East Coast most frequently have southwest flow aloft. However, for the Upper Ohio Valley, the direction of the flow aloft varies as the warm season progresses. Spring and autumn events occur mostly with southwest flow aloft, while summer events usually occur with northwest flow. More than three-fourths of all Upper Ohio Valley events occur with northwest flow aloft. The seasonal transition in the direction of flow aloft is linked to the emergence of a long-wave ridge over the Plains states in summer and its subsequent decay in autumn. Since the ridge typically extends only as far east as the Appalachian Mountains (approximately 78°W longitude), the ridge and its downwind northwest flow seldom affect the East Coast.

For the most part, synoptic conditions during the strong convective events depart very little from the mean summertime conditions. Notable exceptions are 1) extremely high dewpoints in the surface–850-mb layer, and 2) stronger low-level clockwise directional shear (mostly in the boundary layer) and helicity. It was also found that most events occur in proximity to an upper-level jet-stream maximum and/or a diffluence zone.

The mean environmental conditions during strong tornado events show several differences from conditions during rainstorm events. Specifically, the tornado environment has stronger low-level wind shear and helicity than the rainstorm environment. Copious moisture extends to at least 700 mb in the rainstorm environment, whereas the air in the tornado environment becomes drier than normal in the 850–700-mb layer. Tornadoes favor the warm side of synoptic-scale thermal boundaries, while rainstorms occur with roughly the same frequency on either side of the boundary. Lastly, the upper-level (700–200 mb) wind speeds in the tornado environment are about one-third faster than those in rainstorm environments. Surprisingly, rainstorm wind speeds are about one-third faster than the summertime norm.

Abstract

Storm-related literature spanning 45 years (1942–1986) was examined to 1) establish a climatology of strong tornadoes and intense rainstorms for the Mid-Atlantic region, and 2) identify the key environmental features that distinguish the days producing strong convective events. Ninety-three strong tornado events (F3 or greater) and 63 intense rain events (19 cm or more in 12 h or less) were found for the 45-year period and were investigated.

The diurnal and monthly distributions of the tornado and rainstorm events correspond well with other known climatological studies of similar events. Analysis of the upper-level flow patterns during the events shows that the Mid-Atlantic region is affected by two distinct regimes during the warm season. Events near the East Coast most frequently have southwest flow aloft. However, for the Upper Ohio Valley, the direction of the flow aloft varies as the warm season progresses. Spring and autumn events occur mostly with southwest flow aloft, while summer events usually occur with northwest flow. More than three-fourths of all Upper Ohio Valley events occur with northwest flow aloft. The seasonal transition in the direction of flow aloft is linked to the emergence of a long-wave ridge over the Plains states in summer and its subsequent decay in autumn. Since the ridge typically extends only as far east as the Appalachian Mountains (approximately 78°W longitude), the ridge and its downwind northwest flow seldom affect the East Coast.

For the most part, synoptic conditions during the strong convective events depart very little from the mean summertime conditions. Notable exceptions are 1) extremely high dewpoints in the surface–850-mb layer, and 2) stronger low-level clockwise directional shear (mostly in the boundary layer) and helicity. It was also found that most events occur in proximity to an upper-level jet-stream maximum and/or a diffluence zone.

The mean environmental conditions during strong tornado events show several differences from conditions during rainstorm events. Specifically, the tornado environment has stronger low-level wind shear and helicity than the rainstorm environment. Copious moisture extends to at least 700 mb in the rainstorm environment, whereas the air in the tornado environment becomes drier than normal in the 850–700-mb layer. Tornadoes favor the warm side of synoptic-scale thermal boundaries, while rainstorms occur with roughly the same frequency on either side of the boundary. Lastly, the upper-level (700–200 mb) wind speeds in the tornado environment are about one-third faster than those in rainstorm environments. Surprisingly, rainstorm wind speeds are about one-third faster than the summertime norm.

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